1. Field of the Invention
The present invention relates to a method of manufacturing a low cost, lightweight hollow cam shaft.
2. Description of the Relevant Art
Recently, there has been a concerted effort to decrease the weight of internal combustion engines in order to improve the specific fuel consumption on one hand, and to increase maximum speed and maximum power on the other hand. Consequently, cam shafts of decreased weight and improved wear-resistivity have been sought. In previous attempts to satisfy these requirements, various hollow cam shafts have been developed which are lighter than solid crank shafts and have high wear-resistivity, such as cam shafts using separate journal pieces and cam members made from sintered materials and mounted on a hollow steel pipe member.
One of the lightweight conventional cam shafts is shown in FIG. 1, wherein on hollow steel pipe member 1, there are mounted several journal pieces 2 and cam members 3 (one of each being shown in cross section). Typically, the outer diameter of each of the cam members 3 is larger than the outer diameter of each of the journal pieces 2. Also, in this type of cam shaft, the difference in outer diameters between pipe member 3 and journal piece 2 is small. Because the resulting thickness of each journal piece 2 is very small, it is very difficult to form the journal piece by a sintering process and to mount it onto the pipe member. Journal pieces formed by a sintering process are likely to be broken.
If the thickness of the journal pieces is made larger so as to overcome these drawbacks, the outer diameter of the pipe member is required to be made smaller, decreasing the mechanical strength, thereof. Thus, prior to the present invention, in order to decrease the outer diameter of the pipe without decreasing the mechanical strength a smaller inner pipe diameter was required which unavoidably increased the thickness of the pipe member. Any increase in the thickness of the pipe member causes a corresponding increase in both the weight and cost of the cam shaft.